The present invention relates to a method and/or architecture for implementing amplifiers generally and, more particularly, to a method and/or architecture for implementing a direct coupled distributed amplifier.
Direct coupled amplifiers operating beyond 50 GHz are needed for test instrumentation, fiber optic systems, and satellite communication systems. Conventional lumped element analog feedback topologies can easily obtain direct coupled performance, but are typically limited in bandwidth by the RC parasitics of the transistors and interconnects. Conventional distributed amplifier topologies achieve wide bandwidths by absorbing transistor and interconnect parasitics into the design, but are difficult to directly DC couple. Conventional distributed amplifier topologies also tend to implement large termination bypass capacitors and are often spatially inefficient.
Several conventional approaches are presently used to satisfy the need for high gain-bandwidth direct-coupled amplification. One approach is a resistive feedback lumped element analog topology, such as the Darlington feedback amplifier. However, the performance of a Darlington feedback amplifier ultimately impaired by device and interconnect parasitics. Another approach used is to directly cascade distributed amplifiers using source/emitter follower and/or diode level shift circuits to obtain higher gain-bandwidth. However, directly cascaded conventional distributed amplifiers suffer from the expense of size, stability, poor inter-stage voltage standing wave ratio (VSWR) and/or group delay and/or gain ripple.
Referring to FIG. 1, a conventional Darlington amplifier 10 is shown. The conventional Darlington amplifier 10 is noted for having wide bandwidth capability. Because of the direct-coupled topology, the conventional Darlington amplifier 10 allows gain performance down to DC. The upper bandwidth performance is, however, ultimately limited by device and interconnect parasitics which have a profound impact at microwave frequencies.
In order to achieve high gain-bandwidth, distributed amplifiers have been directly coupled through the use of emitter or source followers and/or diode level shifter circuits. FIG. 2 illustrates a conventional emitter or source follower (or circuit) 20. FIG. 3 illustrates a conventional diode level shifter (or circuit) 30. The circuit 20 and the circuit 30 provide DC coupling between distributed amplifiers at the expense of higher power consumption, larger size (about twice the die area of a single stage distributed amplifier) and potential instability and gain ripple problems. In addition, conventional distributed amplifiers incorporate large termination bypass capacitors (not shown) to obtain base band performance and thus implement several off-chip components.
Referring to FIG. 4, a performance of a traditional analog direct coupled feedback amplifier is shown. Analog direct coupled feedback amplifiers are characteristic of flat responses down to baseband because of the absence of frequency limiting capacitor and inductor networks. A forward transmission (i.e., insertion) scattering parameter (i.e., S21) can have a clean performance from baseband to the 3-db BW requirement of greater than 35 GHz for a 40 Gb/s operation. An input reflection scattering parameter (i.e., S11) should also be consistent over the bandwidth. In particular, the parameter S11 should reflect a constant impedance across the band to ensure flat transimpedance performance. The output reflection scattering parameter (i.e., S22) should be better than 10 dB across the band. The log plot shows that the direct coupled topology achieves consistent baseband behavior.
Referring to FIG. 5, a linear frequency performance of the traditional analog direct coupled topology is shown. Gain and input return-loss degrade at higher frequencies due to device and interconnect parasitics. Specifically, capacitive parasitics tend to roll-off the forward transmission gain (i.e., S21) as the frequency increases. The input return-loss (i.e., input reflection coefficient S11) reduces the input impedance at higher frequencies. The output reflection coefficient (i.e., S22) remains relatively constant at all frequencies.
It would be desirable to obtain the consistent baseband performance of a direct coupled device while being implemented in a compact area while achieving high frequency bandwidth performance similar to performance obtained from distributed amplifier designs.
The present invention concerns an apparatus comprising an input stage, an output stage, a bias circuit and a feedback circuit. The input stage may be configured to generate a plurality of intermediate signals in response to an input signal. The output stage may be (i) DC coupled to the input stage and (ii) configured to generate an output signal in response to the intermediate signals. The output stage generally comprises a plurality of distributed amplifiers each configured to receive one of the intermediate signals. The bias circuit may be (i) connected between the input stage and the output stage and (ii) configured to adjust an input impedance of the input stage.
The objects, features and advantages of the present invention include providing a direct coupled distributed amplifier that may (i) have a distributed common emitter input stage directly DC coupled to a second common emitter, (ii) implement a Darlington distributed stage that is scalable, (iii) provide cascadable wide band performance, (iv) have scalability to N number of directly DC coupled distributed amplifier sections for achieving higher direct-coupled gain, (v) have a self-biasing portion that incorporates broadband active load, (vi) have termination that enhances and maintains lower frequency response, (vii) have a biasing portion that may include feedback between the active components of distributed stages, (viii) have a biasing portion that provides gain-temperature compensation, (ix) implement an application of the Darlington pair as a transconductor cell of a microwave distributed amplifier topology, and/or (x) be an application of direct-coupled feedback between 2 or more stages of gain within a distributed amplifier.